(1) Field of the Invention
The present invention relates to the movement of a flexible media from one reel or spool to another, during the performance of some operation on the media during the transfer, and particularly to the exercise of control over the velocity of and tension imposed upon the media during the movement thereof. More specifically, this invention is directed to an electronic control for a reel-to-reel drive system, for example a tape cassette drive, which maintains a constant tape velocity at a recording or pick up head during normal operation, maintains a constant safe level of tension on the tape, quickly brings the tape to speed upon receipt of a start command and quickly stops the tape movement upon receipt of a stop command while preventing the transient tension from approaching the yield point of the tape. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention has been found to be particularly well suited for use in a tape transport intended for use in cassette recorders, particularly cassette recorders intended for employment as digital magnetic memories in data processing apparatus. The environment of a digital cassette tape drive imposes a number of requirements on the tape transport. Thus, in order to maximize the data storage capability of the tape, the linear velocity of the tape at the record/playback head must be constant. The maintainence of the desired constant linear velocity is rendered difficult because the radial hub velocity varies as a function of the instantaneous combined diameter of the takeup reel and the tape stored thereon. Thus, the problem becomes that of finding a way to determine and subsequently maintain constant the actual linear type velocity. The solution to this problem must be capable of being implemented reliably, inexpensively and with volumetric efficiency. An associated problem, which must be addressed to insure that data will be recorded and retrieved with a high degree of accuracy and that damage to the recording media will be minimized, is the maintainence of a constant and safe level of tension on the tape in the region between the pay-out (supply) reel and takeup reel. Failure to maintain a constant tape tension will, for example, result in uneven application of the tape against the record/playback head which results in errors in the data processing.
Continuing to discuss the requirements of a tape transport which is suitable for use in a digital cassette tape drive, as the tape is transferred from reel to reel there will be inertial imbalances. Since the tape should be brought up to its constant working velocity as quickly as possible, and the system must remain stable, compensation must be provided for these inertial changes.
Yet another highly desirable characteristic of a tape transport for digital data storage applications is minimal stop time. In other words, the tape motion must be arrested as quickly as possible but this must be accomplished gently so that the transient tension does not approach the yield point of the tape thus causing permanent damage.
Prior art tape transports have not addressed all of the above-briefly discussed problems simultaneously and provided a solution thereof which is characterized by a high degree of reliability, moderate expense and good volumetric efficiency. Solutions to some of the above-discussed problems have been proposed but apparatus embodying the solutions have been characterized by one or more deficiencies.
Cassette drives for memory storage in computer systems intially employed either one or two capstans with associated drive motors and single or bidirectional speed control was sought through the exercise of control over the capstan drive motors. These early digital cassette tape drives additionally had a pair of reel drive motors for winding the tape in the cassette. The reel drive motors were also used for fast forward and rewind. The deficiencies of capstan drive systems include complexity, cost and excessive tape wear. For example, capstan drive machines meter the tape by capturing it between a pinch roller and the output shaft of the associated capstan drive motor. This results in actual tape contact which reduces tape life through wear and increases system contaminants which in turn, reduces data integrity.
More recent tape transports designed for digital applications have eliminated the capstan motors and regulate tape speed by exercising control over the reel drive motors. In order to overcome the above-discussed problem of tape velocity variation with takeup reel diameter, various techniques have been proposed for measuring actual tape velocity at the recording head. Thus, for example, it has been proposed to use a pre-recorded clock track written on the tape and to generate a tape speed signal by monitoring this clock track. While this approach is operable, it significantly reduces the data storage capacity of the tape. It has also been proposed to employ a low inertia tachometer or optical encoder which is attached to the shaft of a tape idler wheel which contacts the tape at a point adjacent the record/playback head. The velocity signal provided by the tachometer or encoder, being proportional to tape velocity, may be used as a feedback signal in a drive motor servo loop. The employment of a tape idler wheel and associated velocity sensor, however, increases the system complexity by adding another mechanical assembly and also increases system inertia. Also, as in the case of the capstan drive systems mentioned above, the contact between the idler wheel and the oxide side of the tape results in tape wear, an increase in system contaminants and thus a reduction in data integrity. It has additionally been proposed to employ the back emfs of the reel drive motors as a measure of motor speed and to subsequently approximate tape velocity as a function of the thus sensed "speeds".
The typical prior art approach to controlling tape tension is to simply permit the motor which drives the payout reel to be rotated by the motion imparted to the tape whereupon the motor will act as a generator and produce a voltage. This voltage is applied across a load resistor thereby resulting in a current flow through the motor which creates a retarding torque. However, since the angular velocity of the rotor of the payout motor varies with the amount of tape on the payout reel, this approach results in the retarding torque, and thus also the tension, varying with position. In fact, employing a standard Philips cassette, the tension will change by (2.4).sup.2 and such a large change is totally unacceptable. It has also been proposed to employ, in those systems where payout motor speed is calculated from a measure of back emf, a function generator which will provide an output signal which may be utilized to control the power applied to the payout reel motor for the purpose of insuring that the braking will not be excessive and damage the tape. Such controls, however, do not maintain substantially constant tension but rather may be looked upon merely as rather complex safety systems. A further prior art method of tension control adjusts the voltage applied to the payout motor as a function of the speed of the take-up motor in an open-loop system, the speed of the drive motor being directly proportional to the torque of the payout motor with constant tension. Tape tension is not actually measured or calculated and this method of tension control is totally ineffective during starting.
The problem of inertial imbalance, while known, has largely been ignored in the prior art.
The achievement of the stopping of tape motion quickly but without imposing unduly high transient tension on the tape has similarly received little attention in the prior art. Typically, when it is desired to stop the tape, the windings of the takeup reel drive motor are short circuited and some control is exercised over the payout reel drive motor. In the prior art it has been customary to apply either a fixed voltage or a constant current to the payout motor during stopping. The use of a fixed voltage, which is applied either for a fixed time period or until the tape velocity feedback indicates that the tape has almost stopped, is the easiest technique to implement. However, since the generated back emf changes by a factor of 2.4, again considering a standard Philips type cassette, and the effective hub diameter changes by the same factor, the applied stop tension may vary by as much as a factor of five and one half. This is too great a variation and results in long stop times. The use of a constant current produces a constant torque during stopping but the tape tension will still vary by a factor of 2.4. Further, the application of a constant current to the payout motor is a more difficult technique to implement in hardware.